rwsem: Add comments to explain the meaning of the rwsem's count field

It took me quite a while to understand how rwsem's count field
mainifested itself in different scenarios.

Add comments to provide a quick reference to the the rwsem's count
field for each scenario where readers and writers are contending
for the lock.

Hopefully it will be useful for future maintenance of the code and
for people to get up to speed on how the logic in the code works.

Signed-off-by: Tim Chen <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Alex Shi <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Peter Hurley <[email protected]>
Cc: Paul E.McKenney <[email protected]>
Cc: Jason Low <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Link: http://lkml.kernel.org/r/1399060437.2970.146.camel@schen9-DESK
Signed-off-by: Ingo Molnar <[email protected]>

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